The Impact of Gantry Rotations On the Accuracy of IMRT Dose Measurements Using EPIDs

Purpose: The use of electronic portal imaging device (EPID) in IMRT QA has recently gained popularity. This study is to investigate the effect of gantry rotations on the accuracy of measured dose distributions.

Methods:Two Varian LINACs (Clinac-iX and Trilogy) were used to collect EPID data daily over 3-week period. Energy fluencies were measured at gantry angles separated 45° apart (0°,45°,...270°,315°)for both static fields defined by asymmetric jaws and dynamic fields from patient treatment plans. The measured fluencies were converted to 2D dose distributions by EPIDoseTM. The dose distributions obtained at zero gantry angle were used as base lines (BL) for comparison with those measured at non-zero gantry angles. The differences generated from static asymmetric fields were used to identify the mechanical misalignment between the center of EPID and beam axis. These differences were applied as the "shifts" to correct the corresponding IMRT fields. The modified dose distributions were then compared with their BL for error analysis using Gamma Index (1mm/1%).

Results:The rotations of gantry and their associated "sagging" have various effects over the accuracy of measured dose distributions. The un-shifted Gamma passing rates were between 34.5% and 100%, dependent on the gantry angles. For both machines, the errors were consistently larger for dose comparisons made between gantry angles 135° -225°,with the lowest average of 44.2% at angle 225°. There have also been consistently high passing rates (>95%) between angles 315°-45°(clockwise). Once the pre-determined "shifts" were applied, all the matching statistics were improved to over 90%.

Conclusion:Gantry rotation can exert effect on the accuracy of measured dose distributions. Proper shifts should be used, especially for gantry angles 135° -225°, to reduce the effect caused by the misalignment between EPID/beam axis. Blindly use of "best fit" available from QA software would result in hiding true errors.